memcg: get_mem_cgroup_from_mm()

Instead of returning NULL from try_get_mem_cgroup_from_mm() when the mm
owner is exiting, just return root_mem_cgroup.  This makes sense for all
callsites and gets rid of some of them having to fallback manually.

[fengguang.wu@intel.com: fix warnings]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 14 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c3b674f9774f..87c3ec37dd26 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1071,7 +1071,7 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
         return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
 }
 
-struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
+static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 {
         struct mem_cgroup *memcg = NULL;
 
@@ -1079,7 +1079,7 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
         do {
                 memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
                 if (unlikely(!memcg))
-                        break;
+                        memcg = root_mem_cgroup;
         } while (!css_tryget(&memcg->css));
         rcu_read_unlock();
         return memcg;
@@ -1475,7 +1475,7 @@ bool task_in_mem_cgroup(struct task_struct *task,
 
         p = find_lock_task_mm(task);
         if (p) {
-                curr = try_get_mem_cgroup_from_mm(p->mm);
+                curr = get_mem_cgroup_from_mm(p->mm);
                 task_unlock(p);
         } else {
                 /*
@@ -1489,8 +1489,6 @@ bool task_in_mem_cgroup(struct task_struct *task,
                         css_get(&curr->css);
                 rcu_read_unlock();
         }
-        if (!curr)
-                return false;
         /*
          * We should check use_hierarchy of "memcg" not "curr". Because checking
          * use_hierarchy of "curr" here make this function true if hierarchy is
@@ -3617,15 +3615,7 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
         if (!current->mm || current->memcg_kmem_skip_account)
                 return true;
 
-        memcg = try_get_mem_cgroup_from_mm(current->mm);
-
-        /*
-         * very rare case described in mem_cgroup_from_task. Unfortunately there
-         * isn't much we can do without complicating this too much, and it would
-         * be gfp-dependent anyway. Just let it go
-         */
-        if (unlikely(!memcg))
-                return true;
+        memcg = get_mem_cgroup_from_mm(current->mm);
 
         if (!memcg_can_account_kmem(memcg)) {
                 css_put(&memcg->css);